Particle monitoring devices using visual, audio, electronic parameters are used to detect particles in moving fluids. Vacuum cleaner lines have been provided with particle or dirt traps which function as settling chambers for receiving the relatively heavy particles moving in the air stream. An example of this vacuum cleaner trap is shown in U.S. Pat. No. 3,267,650. Some vacuum lines have been provided with settling chambers for observing and separating heavy objects from a moving air stream. An example of this structure is shown in U.S. Pat. No. 944,779. The use of a visual window, or other visual indicating means, for the purposes of monitoring the amount of particles in a moving air stream, has proven ineffective, as the window material, glass or other transparent medium becomes clouded. Efforts to overcome the inadequacies of the visual monitoring systems have been made by the use of a small circular rigid diaphragm, which will produce some audible sounds to provide an indication of dust or dirt in the air stream. An example of this structure is shown by McClatchie in U.S. Pat. No. 1,633,598.
Tests have shown that the McClatchie particle monitoring device operates at a resonant frequency that is substantially higher than the frequency of the highest human audio sensitivity. The McClatchie device peaks at 8 KH.sup.2, has a relatively low sound output and is inoperative in slow moving air as it does not produce an audible output signal. The diaphragm of the McClatchie device collects dirt particles on the impaction surface fairly quickly. The accumulation of particles on the inside surface of the diaphragm dampens the sound output. In order to provide for an effective monitoring of the particles, the McClatchie diaphragm must be removed and cleaned as it does not have self-cleaning characteristics.